Quantum Physics

Title:
Conjugate Degradability and the Quantum Capacity of Cloning Channels

Abstract: A quantum channel is conjugate degradable if the channel's environment can be
simulated up to complex conjugation using the channel's output. For all such
channels, the quantum capacity can be evaluated using a single-letter formula.
In this article we introduce conjugate degradability and establish a number of
its basic properties. We then use it to calculate the quantum capacity of N to
N+1 and 1 to M universal quantum cloning machines as well as the quantum
capacity of a channel that arises naturally when data is being transmitted to
an accelerating receiver. All the channels considered turn out to have strictly
positive quantum capacity, meaning they could be used as part of a
communication system to send quantum states reliably.